Differential probes are primarily used for detecting ferromagnetic bodies hidden in the earth or in water, such as bombs, mines, ships, ship parts or the like. Such bodies are located by evaluating the disturbance caused by such bodies of the otherwise homogeneous magnetic field of the earth, i.e., the earth field gradient. In many cases, harmonic-wave magnetic field sensors are used for this purpose in which the transducer consists of a soft-magnetic core element of high magnetic permeability. The core is magnetized to full saturation by a magnetic alternating field of a given frequency. An external magnetic field occurring in the direction of the core element results in a dissymmetry of the magnetization characteristic of the core element thereby producing in the winding surrounding the core element an electric voltage of an amplitude proportional to the field strength of the external magnetic field and of a frequency consisting in an even-numbered harmonic wave of the frequency of the exciting alternating field. Although it is true that harmonic wave magnetic field probes are useful for this purpose, other magnetic field sensors may on principle also be used in a differential probe, and, in particular, in so-called search instruments.
Frequently, the above-mentioned differential probes must resolve magnetic field differences in the range of 1.times.10.sup.-9 Tesla (1 nano Tesla), while under the influence of a field strength of 50,000 nano Tesla. In the case of a search device, this may for instance be the influence of the magnetic earth field. However, this means that exacting demands must be placed on the parallelism of the magnetic axes of the magnetic field sensors used in relation to each other and also in relation to an imagined straight line for the entire probe; otherwise, a difference in the magnetic fields could be falsely indicated in the case of a rotation of the differential probe about its longitudinal axis, for example. The angular deviation from parallelism would have to be kept below 4 seconds of arc, i.e., 2.times.10.sup.-5 radians.
In search applications, the sensitivity of the differential probe increases as the base distance between the magnetic field sensors increases. The upper limit of the base distance is determined by the temperature and aging effects provoking a maladjustment of the parallelism of the magnetic axis of the sensors. Presently, this upper limit is approximately 0.5 meters.
A magnetic field differential probe of the general type referenced above is set forth in U.S. Pat. No. 3,982,179. The differential probe of this patent has each magnetic core element fastened onto two stretched wires extending at a specified distance and parallel to each other. One end of the wires is provided with means for maintaining the parallelism which enables the exact adjustment of the parallelism of the magnetic core element in two planes vertical to each other. The two wires are generally part of a continuous wire formed by a return means into a U-shaped loop and maintained under mechanical tension by a spring.
As compared to differential probes formerly used, the probe of this patent offers a considerable advantage. While the former differential probes had their magnetic field sensors installed in the upper and lower ends, respectively, of a probe tube so that any distortion of the probe tube provoked a notable maladjustment of the parallelism of the magnetic axes, this error source is eliminated.
However, there still remain other possible detrimental effects upon the parallelism which are important. Different degrees of extension of the material of the return means and the material between the points where the free ends of the U-shaped wire loop are clamped, lead to non-parallelism. The same applies even more to a possible rotation of the return means about the longitudinal axis of the differential probe. Such a rotation for a given angle causes the two ends of the wires in the return means to be dislocated in opposite senses, and this, in turn, results in a deviation from parallelism which is greater the larger the diameter of the return means becomes. The only way to avoid this latter error is to provide a correspondingly sturdy and weighty design of the return means, but this necessarily renders handling more difficult and increases cost. The means for maintaining the parallelism, which necessarily must be constructed to cover a relatively wide range of adjustment, constitute an additional risk of changes from parallelism due to temperature influences and shocks. Still further, the two means for maintaining parallelism require finely adjustable slides with carefully finished guide faces, which renders them rather expensive. If the deviation from parallelism produced by the gravity forces resulting from the slack of the two wires when the differential probe is in the horizontal position, is to be kept within narrow limits, the tension must be very high. But the high tension, in turn, means that the tension forces in the two wires must be well balanced by the use of a freely turning return roller, for example. However, in this case the shock-proof suspension of the roller can be achieved only at great cost. The desire to reduce the slack of the wires leads to the use of materials for the latter which combine the properties of light weight, low extension and strength. However, as such materials are rather brittle, they require a wide radius of curvature, and this, in turn, leads to undesirably large diameters for the return means.